This disclosure relates to receivers for orthogonal frequency division multiplex (OFDM) modulation and more particularly, to a channel equalizer capable of reducing multi-path channel noise.
Orthogonal frequency division multiplex (OFDM) modulation is a digital modulation transmission method using multiple carrier frequencies (e.g., in the environment of wireless (or mobile) communications), that is strongly immune against multi-path and fading channels. The OFDM modulation also has excellent spectrum efficiency, so it is being widely used and considered for various wireless communications. For example, the OFDM technique is being advantageously employed in receivers for European digital video broadcasting televisions (DVB-T).
In a wireless transmission path along which a signal sent from a transmission station arrives at a receiver through space, a radio wave is usually reflected on or scattered by spatial impediments such as large constructions, or mountains. Because of that, the receiver may further receive radio waves (i.e., indirect waves) reflected on such obstacles, in addition to a wave signal received directly from the transmission station, i.e., a direct wave signal. The reflected and direct waves are usually received at different times at the receiver, affecting the demodulation process in the receiver. As a result, the receiver may be demodulating an additively distorted signal, not the originally transmitted signal.
It is desirable to reduce the effects of the noise in a multi-path channel communication environment by modifying the receiver. Thus, a typical receiver used therein employs an equalizer to compensate for distortion caused by the aforementioned multi-path channel effect. The equalizer is typically designed having an adaptive function that follows variations of transmission path environments without a fixed operational characteristic.
Meanwhile, a coherent OFDM receiver usually has an equalizer that estimates sub-channels to compensate for the distortion of the channel and adopts the inverse of the characteristic of each of the estimated sub-channels as equalizing coefficients. However, it is difficulty to correctly estimate channels due to delay times of signals received by an OFDM receiver through multi-path channels.